Why our speed limits sometimes change:  USA Today: Speed limits: Tried and true, or invalid?

From USA Today

Speed limits: Tried and true, or invalid?

http://bwne.ws/2bWqFsf

Follow the speed limit.It’s a simple task most drivers do successfully every day. But there’s still a small part of the driving population that travels above or below it. We might think these drivers are just not abiding by a law that’s been set for a reason, but what if instead of the driver, the speed limit is incorrect?In St. Clair County, it’s possible, because some speed limits were set in the 1940s and haven’t changed since. The most recent speed limit set on a county road was St. John Drive in 2005 in Clay Township.This was just before the passage of Public Act 85 of 2006, which mandates that cities, townships and villages complete speed studies or access-point counts to justify their speed limits.